Atomic 'Billiards Game' Illuminated by Laser

An international collaboration has observed the occurrence of nonsequential double ionization in argon atoms. In successfully confining the ionization to a single recollision and excitation event, the process can be traced on attosecond timescales.

Artist's view of nonsequential double ionization. The 3-D plots on the circle were obtained from experimental data and show how the velocities of the two electrons change with the electric-field evolution of the ionizing pulse. The plot in the center is the sum of the single measurements. From these data, the scientists can reconstruct the detailed process of the double ionization. (Image: Christian Hackenberger/LMU)

When intense light is shone on a neutral atom, the photons in the light interact with the atom, imparting energy that excites an electron. The electron is ejected from the atom with an energy related to that of the instigating photon, and the atom assumes a negative charge, becoming an ion. Classically, ionization tends to emit electrons only sequentially. A -2 atom must be made from a -1 emitting an electron, or a -3 gaining one. However, as with many other classical laws, this breaks down at the quantum level.

Led by scientists at Max Planck Institute for Nuclear Physics and the Laboratory for Attosecond Physics at Max Planck Institute of Quantum Optics, the investigators shot 4-fs-long laser pulses onto argon atoms. The light wave was two cycles long, so there were two-wave maxima and two-wave minima, or two cycles. As one might expect, most atoms in this laser field were singly ionized. However, 1 in 1000 of the atoms underwent nonsequential double ionization. As the first electron was ejected from the atom, the electric field of the laser shot it back into the atom. There, it collided with another electron and promoted it to an excited state. Just before the second laser maxima interacted with the atom, the newly excited electron was ejected. The entire process can be compared to a game of billiards, where, after a collision, a ball brings another one in motion.

A scientist from the Laboratory for Attosecond Physics at Max Planck Institute of Quantum Optics works with the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) apparatus, which is used to perform the experiments on double ionization. (Image: Thorsten Naeser/LMU)

The observations made from this process provide insight into the quantum dynamics of a laser-driven multielectron system, essential in studying matter-light interactions as well as how electrons interact with each other during chemical processes.

The research was reported in the May 8 issue of Nature Communications.

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